1. Field of the Invention
This invention relates to a circuit and method for processing the output of an implanted sensing device for detecting the presence or concentration of an analyte in a liquid or gaseous medium, such as, for example, the human body. More particularly, the invention relates to a circuit and method for processing the output of an implanted fluorescence sensor which indicates analyte concentration as a function of the fluorescent intensity of a fluorescent indicator. The implanted fluorescence sensor is a passive device, and contains no power source. The processing circuit powers the sensor through inductively coupled RF energy emitted by the processing circuit. The processing circuit receives information from the implanted sensor as variations in the load on the processing circuit.
2. Background Art
U.S. Pat. No. 5,517,313, the disclosure of which is incorporated herein by reference, describes a fluorescence sensing device comprising a layered array of a fluorescent indicator molecule-containing matrix (hereafter xe2x80x9cfluorescent matrixxe2x80x9d), a high-pass filter and a photodetector. In this device, a light source, preferably a light-emitting diode (xe2x80x9cLEDxe2x80x9d), is located at least partially within the indicator material, such that incident light from the light source causes the indicator molecules to fluoresce. The high-pass filter allows emitted light to reach the photodetector, while filtering out scattered incident light from the light source. An analyte is allowed to permeate the fluorescent matrix, changing the fluorescent properties of the indicator material in proportion to the amount of analyte present. The fluorescent emission is then detected and measured by the photodetector, thus providing a measure of the amount or concentration of analyte present within the environment of interest.
One advantageous application of a sensor device of the type disclosed in the ""313 patent is to implant the device in the body, either subcutaneously or intravenously or otherwise, to allow instantaneous measurements of analytes to be taken at any desired time. For example, it is desirable to measure the concentration of oxygen in the blood of patients under anesthesia, or of glucose in the blood of diabetic patients.
In order for the measurement information obtained to be used, it has to be retrieved from the sensing device. Because of the size and accessibility constraints on a sensor device implanted in the body, there are shortcomings associated with providing the sensing device with data transmission circuitry and/or a power supply. Therefore, there is a need in the art for an improved sensor device implanted in the body and system for retrieving data from the implanted sensor device.
In accordance with the present invention, an apparatus is provided for retrieving information from a sensor device, comprising an internal sensor unit for taking quantitative analyte measurements, including a first coil forming part of a power supply for said sensor unit, a load coupled to said first coil, and a sensor circuit for modifying said load in accordance with sensor measurement information obtained by said sensor circuit; an external unit including a second coil which is mutually inductively coupled to said first coil upon said second coil coming into a predetermined proximity distance from said first coil, an oscillator for driving said second coil to induce a charging current in said first coil, and a detector for detecting variations in a load on said second coil induced by changes to said load in said internal sensor unit and for providing information signals corresponding to said load changes; and a processor for receiving and processing said information signals.